Endogenous type C virus expression in the rat myogenic cell line, L8, has been characterized. Release of virus was measured during the proliferative and differentiative phase of cultivation. Virus production was unaffected by blocking terminal differentiation but was diminished by inhibitors of DNA synthesis. Exogenous infection of L8 cells with 2 helper independent type C viruses has been shown to block myogenic differentiation. The block appears to be late during differentiation and is directly related to virus replication.